The present invention relates to an ignition coil, of the type intended to be mounted on a spark plug for the individual electrical supply of this spark plug.
Such coils, comprising a magnetic core, a tubular secondary holder inside which is placed the core and onto which the secondary wire is wound, this assembly being placed in the primary holder onto which the primary wire is wound, are known. A slit metal tubular shell, for avoiding eddy currents, is placed around the assembly in order to ensure the return of the magnetic flux and the whole assembly being contained in a plastic casing.
However, it is known that these coils must have a small diameter since they are housed in wells of the engine""s cylinder head, immediately above the spark plugs that they supply.
It is important for the radial distance between the primary and the secondary to be as large as possible. This is because, when this distance increases the capacitance of the secondary decreases and the breakdown voltage increases. Furthermore, in the case in which, because of the thermal shocks to which the spark plug could be subjected, microcracks occur, it is advantageous to increase the arc breakdown path.
The problem stems from the fact that the external diameter of the casing is fixed by the diameter of the wells. Moreover, most of the dimensional parameters of the coil are set by its electrical properties. The casings must therefore be as thin as possible, and hence they are difficult to produce and are expensive in terms of scrap.
The present invention aims to remedy these drawbacks.
For this purpose, the subject of the invention is an ignition coil, of the type intended to be mounted on a spark plug for the individual electrical supply of this spark plug, comprising an internal secondary winding, an external primary winding, a flux return shell and a casing, characterised in that the said casing surrounds only the secondary winding, the primary winding being wound onto the casing, on the outside of the latter, and being itself surrounded by the flux return shell.
Because the primary is wound directly onto the casing, the primary holder may be dispensed with. Alternately, if the element on which the primary winding is wound is considered to be the primary winding holder, then this element forms a portion of the outside of the ignition coil and a casing is no longer necessary. However, in this disclosure, the element that supports the primary winding, which also forms a portion of the outer surface of the ignition coil and surrounds the core and secondary winding, is usually referred to as the casing. The casing can therefore be thick and consequently manufactured much more easily. Furthermore, it forms part of the insulation and greatly reduces the risks of flashover between the high-voltage terminal and the primary winding.
In one particular embodiment, the casing is in the form of a body of revolution, the external surface of which has a first annular recess, for accommodating the flux return shell, and a second annular recess, axially and radially more deeply included in the first, for accommodating the primary winding.
One particular embodiment of the invention will now be described, by way of non-limiting example, with reference to the appended schematic drawings in which: